1. Field of the Invention
The present invention relates generally to a polarization conversion and recycling method that features a polarization converting apparatus using a uniquely designed half-pyramid shaped reflector with multi-reflection to achieve a desirable polarization rotation function. The present invention further relates to polarization conversion and recycling systems using the same.
2. Description of the Prior Art
Polarized light sources are utilized in various applications such as LCD (Liquid Crystal Display) and LCOS (Liquid Crystal On Silicon) projection systems. Un-polarized light source can be decomposed by a polarizing beam splitter (PBS) into a linear s-polarization component and an orthogonal p-polarization component.
A commonly used method of producing polarized light for a LCD projection panel comprises the use of a PBS prism cube or a reflective PBS plate to separate s and p polarized beams into reflection and transmission, respectively. One linearly polarized component of the light is transmitted by the PBS and directed to the LCD panel, while the orthogonal component is reflected away in different direction. The regular PBS could only be used near 45 degrees incidence with small numerical aperture. Another kind of PBS, the wire grid PBS, has attracted special interest because it can be used in a perpendicular incidence with large angle incident cone, as disclosed in U.S. Pat. No. 6,208,463 and U.S. Pat. No. 6,243,199.
It is evident that conventional reflective polarizers with no polarization conversion and recycling device attached are inefficient, due to the fact that, during s-p polarization split, severe loss of light occurs and only a maximum of one-half of the available light from the source can be converted to polarized light for transmission.
Many efforts have been made to increase the efficiency of polarization conversion and the results can be found in the prior arts and commercial products. Some of the polarizing converting systems incorporate a waveplate with polarizing beam splitter to make use of both polarizations. For example, by introducing a half waveplate, one could convert a p-beam into a s-beam or vice versa to recover otherwise wasted component. A mirror or a prism has to be employed to reflect the converted beam in the same direction as the other beam does. Examples can be found in U.S. Pat. No. 6,046,856 and U.S. Pat. No. 5,884,991.
Another commonly proposed approach involves the implementation of units that incorporate a plurality of prisms arranged in an array with PBS coating or reflecting surfaces formed between the prisms. Light from the source is directed towards the prisms by a multi-lens optical array. One polarization is passed directly through the prisms while the other polarization is directed sideways towards the mirror surfaces between the prisms. Mirrors redirect the other polarization again into the useful beam. Waveplate strips, arranged on the exiting surfaces, rotate the other polarization so that it is the same as the passed polarization. The width of the elements in the lens array is twice the width of the PBS prism elements so that the structure can make full use of the illumination area. Implementations of such mechanism can be found in U.S. Pat. No. 6,337,759 and U.S. Pat. No. 6,154,320.
In summary, previous work shows that almost all current polarization conversion systems have to utilize half-wave or quarter-wave phase retardation plates to increase conversion efficiency. However, the methodology of employing waveplates in a polarization converter inevitably leads to some problems. One of the disadvantages is that the waveplates will not work properly if the wavelengths of the illumination beam deviate considerably from the center wavelength. Another disadvantage is that the waveplates are sensitive to changes of the incident angles and the direction of polarization. Furthermore, a waveplate made of polymer is thermally unstable, while although a waveplate made of crystal can tolerate high temperature and rapid temperature change, its cost is usually too high. These issues have restricted the use of waveplate in some practical applications, particularly in those applications where working with a high flux of broadband light and/or a high operation temperature is unavoidable.
In some literature from the prior arts, a variety of proposals have been made to achieve polarization rotation and conversion by using prisms. Among them, U.S. Pat. No. 4,822,150 disclosed an optical device for rotating the polarization of a light beam. In one embodiment of the invention, the device comprises a prism, which is adapted to rotate the polarization of a linearly polarized beam of radiation by 90 degrees and the propagation direction of beam is altered by 90 degrees. U.S. Pat. No. 4,252,410 disclosed a method and apparatus for rotating the polarization vector of a substantially linearly polarized beam through a selected angle by using three or four reflections with the output beam being substantially free of ellipticity. U.S. Pat. No. 5,751,482 disclosed an achromatic polarization rotating right angle prism system for rotating the plane of polarization by 90 degrees. The direction of travel of the beam may be altered by 90 degrees or the beam may continue on in the same line. These findings and derived systems are often too complicated to be used in practical polarization applications.
U.S. Pat. No. 5,777,788 presented a prism polarizer, in the form of a pyramid with its apex located on a line normal to the bottom surface at one of the four corner points, to convert unpolarized light into linearly polarized components without using a half wave plate. The prism polarizer is comprised of two reflecting surfaces and a polarization beam splitting surface (PBS surface) which lies in a bisectional plane of two reflecting surfaces. One of the side surfaces of the pyramid which are in the form of right-angled isosceles triangles defines the light incident surface, and a square bottom surface defines the light emission surface.
The unpolarized light incident upon the light incident surface, passes though the PBS surface and is split by the PBS surface. One p-polarized light component, transmitted through the PBS surface, is reflected by the first reflecting surface toward the light emission surface; and the s-polarized light component, reflected by the PBS, is reflected at right angles by the second reflecting surface toward the light emission surface. Thus, the s-polarized light component is emitted in the same polarization direction and propagates in the same direction as the P-polarized light component. The disclosed device is not only a polarizer to convert un-polarized light to polarized light but also a polarization rotation device to change the direction of polarization.
However, the disclosed prism polarizer is not very suitable for many applications due to its triangular shaped incident surface. To overcome this drawback, the inventor demonstrated a rather sophisticated embodiment by using two prism polarizers adhered together to form a square incident surface, and at least one optical component needs to be added in order to keep light emissions from two prisms propagating in the same direction. Due to its potential complexity, again, this configuration is not very practically useful for many real applications.
To address the problems and drawbacks in prior art polarization conversion systems, the current invention introduces a unique polarization rotation apparatus of a half-pyramid shaped reflector, either a half-pyramid shaped prism reflector or a half-pyramid shaped hollow reflector, which can be assembled and integrated in a polarization conversion system to achieve the required polarization rotation, conversion and recycling without using a wave plate. Compared to polarization rotation devices in the prior art which are utilizing wave plates, the invented polarization rotation apparatus has advantage of not being sensitive to wavelength variation of the light source, to temperature changes and to polarization alignment errors. Compared to other prism-based polarization rotation devices disclosed in the prior art, the invented polarization rotation apparatus has advantage of simplicity in structure, compactness in size, and flexibility to be assembled in various configurations.